In a chromatograph, an inert carrier substance (gas, liquid or supercritical fluid) is passed continuously at a controlled rate and temperature through a column consisting of a permeable stationary material contained in a tube. A sample containing compounds to be analyzed is injected into the carrier flow where it enters the column. Due to interaction with the stationary material of the column, various components of the sample travel along the column at different rates, and emerge at different times. Emerging compounds can be detected by any of a variety of methods, such as mass spectroscopy or flame ionization. Detector response can be continuously recorded, so that each emerging component shows up as a peak on the recording. The relative time of emergence of components of a mixture provides information as to the chemical nature of the compounds they represent, while the size of the peaks suggests the concentrations of the compounds.
Thus, it should be appreciated that chromatography may be utilized to (1) separate various compounds in the sample from each other, thereby allowing individual analysis by other methods, and (2) provide some qualitative and quantitative information about each of the compounds thus isolated.
While chromatographic information alone is not likely to be conclusive in identifying separated compounds, except in simple cases, the power of modern chromatographic analysis lies in combining chromatography results with additional sophisticated chemical and/or spectroscopic studies of the chromatographic effluent. In this way, many compounds may be readily and conclusively identified even when present in relatively small concentrations.
To date, chromatographic analysis has proven of particular value in such applications as quantitative analysis of organic pollutants. Still, improvements in both the equipment and techniques involved are desired. More specifically, the compounds of interest in pollutants are typically present in exceedingly low concentrations. Further, in many cases, only a limited amount of the material containing these compounds is available for analysis. In such applications, the amount of compound available can easily be the limiting factor in the accuracy of analysis.
Unstable compounds also present a significant problem as chemical alteration of the compound itself during preparation for analysis can be a source of error. The ideal method to overcome this difficulty would be to perform all the desired analyses in close sequence on a single unique sample of chromatographic effluent. This approach is, however, not possible because (1) some methods of analysis alter or destroy the compounds of interest, or because (2) present chemical devices do not provide for sequential performance of some analyses.
Accordingly, it should be appreciated that in these cases there is presently no ideal solution; that is to say:
Dividing the effluent from a single chromatographic injection into several smaller aliquots for several different analyses may significantly reduce the resolution of these analyses.
Avoiding this problem by using a separate chromatographic injection for each desired analysis is only feasible if the sample material is available in sufficient quantity, and introduces the possibility that the substances analyzed may not be identical.
In either case, there is the possibility that the material presented to one analysis may have undergone chemical alteration so it is no longer identical to the corresponding material subjected to another analysis.
In addition, any loss or inefficiency in collection, concentration, and presentation of these tiny samples for analysis is likely to reduce the resolution of the analysis.
Due to these considerations, any innovation is a significant advancement of art in this field if it: (1) makes it possible to perform previously incompatible analyses on a single effluent aliquot; (2) reduces the likelihood of alteration of compounds of interest in the interface; and/or (3) improves the efficiency of collection, concentration and presentation for analysis. In contrast to conventional approaches described below, the present invention achieves all three of these objectives.
At present, products of chromatographic separation are typically analyzed (1) by continuous monitoring of the effluent stream, or (2) by capturing selected time segments of the effluent stream for later study. For survey of mixtures of unknown composition, continuous monitoring is preferable, whereas for detailed study of selected sample components, capture offers significant advantages.
In continuous monitoring, the chromatographic effluent passes more or less directly into equipment designed for analysis "on the fly". The limitations of this approach in studies, for example, involving the absorbance of electromagnetic radiation (EMR) are reviewed by Griffiths et al (1983) with reference to FT-IR studies. Other "on the fly" methods involve an intermediate continuous sample-processing step, such as vaporization of the effluent from a liquid chromatograph before it enters a spectrometer. Because of the limited time available for observation of a given effluent component, these methods are limited by the speed of data acquisition of the detection system. Consequently, stop-flow techniques have been developed and used with continuous-monitoring systems to increase observation time for selected segments of the effluent stream. These, however, are typically awkward and meet with limited success.
The alternative to continuous monitoring--capture or immobilization of segments of the effluent stream--has the great advantage of allowing lengthy or repeated examination of the capture sample, to achieve a higher signal-to-noise ratio. Capture may be accomplished by (1) trapping in organic solution such as with an impinger, or (2) absorption on organic polymers with large surfaces such as TENEX{.RTM.}. Unfortunately, in ordinary laboratory methods the sample so captured must be transferred to a spectrometer (or other detection system), and one or more additional steps of preparatory processing may be required, each of which takes time and is a possible source of error.
A technique known as cryodeposition has been developed to address these problems. Performed under suitable conditions of temperature and pressure, cryodeposition can tend to isolate the sample compounds from the chromatographic carrier substance, eliminating the need for a separate sample-concentration step. Cryodeposition at extremely low temperatures has the additional advantage that certain compounds which could not be studied in conventional matrices because of decomposition will remain stable for long enough to allow testing.
In the devices described by Conrad et al., in U.S. Pat. No. 4,158,772, and manufactured under the trade name Cryolect by Mattson Instruments, the cryodeposition process occurs on a continuous cryogenic surface in an open vacuum chamber. In these devices, the chromatographic effluent is mixed with an inert and neutral matrix material such as argon gas, and the mixture is directed against a rotatable cooled sample block inside a vacuum chamber, so that a frozen spot or band of the matrix material containing some of the compounds separated by the chromatograph is deposited on the surface of the block. The vacuum chamber is provided with windows to permit spectroscopic examination of the immobilized samples in situ.
Still, despite these advantages, cryodeposition devices have a number of shortcomings:
(1) Because they are only capable of accepting gaseous-phase input, they cannot be used with liquid or supercritical fluid chromatographs without addition of intermediate effluent-processing equipment;
(2) There is little provision made for adjustment of the effluent jet, including its distance or attitude relative to the deposition surface, or of the temperature of the effluent, and no provision for any such adjustments in the course of a chromatographic run. This makes it impossible either to maximize the efficiency of capture of each compound, or to optimize the configuration of the frozen sample deposit for a particular spectrometer, as is crucial with small samples containing very dilute analytes.
With very small samples, maximum signal-to-noise ratio requires sample deposits configured for maximum interaction with the beam of electromagnetic radiation (EMR) from the particular spectrometer that will be used to study them. Roughly speaking, maximum interaction will be achieved when the beam of EMR from the spectrometer is directed at a sample deposit neither larger nor smaller than the beam diameter;
(3) Because deposition is on a locally unconfined surface, such adjustments would in any case have limited effect on the pressure gradient of the jet close to the collection surface, a factor which should be subject to control if optimum collection efficiency and deposit configuration are to be achieved;
(4) Though separate segments of the effluent stream are isolated by deposition at discrete locations on the sample block, there is no provision for recovering them from the device as discrete samples for further analysis by vapor phase methods; and
(5) Because these systems require a large cryostat surface relative to the total deposition surface in an open vacuum chamber the vacuum chamber cannot be conveniently reduced to a small size or configured to facilitate recycling and cleaning.